The present invention relates to a closed market gardening greenhouse and a method for controlling the climate in a closed market gardening greenhouse.
Cultivation under glass takes place worldwide with xe2x80x9copenxe2x80x9d greenhouses. Open greenhouses provide the option of ventilating the greenhouse air by means of ventilating windows. This has the advantage that when insolation is high the surplus heat and/or moisture can be removed in simple manner via ventilation. In the Netherlands it is necessary on a large number of days for the greenhouses to be ventilated for a number of hours.
Optimal culture conditions however require a good balance between insolation, air humidity, greenhouse temperature and CO2 delivery. It is self-evident that the open greenhouse system cannot usually satisfy optimal culture conditions.
Optimal culture conditions for the greatest possible harvest for many crops are as follows:
Temperature between 18 and 24xc2x0 C.
Air humidity between 70 and 90%
Concentration of CO2: 1000 ppm.
From an energy viewpoint an open greenhouse system is not desirable.
The surplus heat in summer is after all removed by ventilation while in winter there is a heat demand.
Because an open greenhouse does not cool actively, the temperature is frequently higher than 24xc2x0 C.
An open greenhouse is supplied with CO2. This CO2 is necessary for the growth of the crop.
Because an open greenhouse must be ventilated often to dispose of the surplus heat and moisture, the supplied CO2 is hereby also lost.
In an open greenhouse the crop will be able to grow quickly particularly in the summer because of high insolation, even though cultivation conditions are not then optimal: too warm and too little CO2. The great quantity of light is not used optimally, mainly because there is a shortfall in the concentration of CO2.
An object of the present invention is to provide an improved market garden greenhouse.
According to a first aspect of the present invention a market garden greenhouse system is provided in which plant products can be cultivated, which market garden greenhouse is closed and substantially not provided with ventilating openings, wherein the market garden greenhouse comprises:
heat regulating means for regulating the heat in the greenhouse, which heat originates from solar energy and a heating system, and/or
air humidity regulating means for regulating the air humidity in the greenhouse.
A closed greenhouse system according to the present invention makes it possible in principle to optimize the greenhouse climate. A closed greenhouse system according to the present invention is understood to mean a greenhouse without ventilating windows which can be opened.
In a closed greenhouse the heat and moisture will be removed without the CO2 concentration being decreased.
With a rapidly responding climate control an optimal balance between the insolation, air humidity, greenhouse temperature and CO2 delivery must be possible at any fluctuation in the insolation.
Advantages of a closed greenhouse according to the present invention are:
the consumption of primary energy (the greenhouse as (closed) solar collector must utilize the insolation to maximum effect) will, according to calculations, be a minimum of 40% lower than in a modern traditional xe2x80x9copenxe2x80x9d greenhouse.
higher cultivation yield because the cultivation conditions such as temperature, air humidity and CO2 concentration can be better controlled and managed. On the basis of model predictions, it is the expectation that the cultivation yield will be a minimum of 20% higher than in a modern traditional open greenhouse.
the use of herbicides/pesticides can be reduced considerably because of the considerable decrease in the chance of crop diseases and infestations; and
saving in water consumption (in a closed system there is the option of collecting and recirculating all the evaporation from the crop; a greenhouse normally uses 500-600 kg/m2 annually).
it is expected that the moment of harvesting can be better controlled. It will be possible to respond better to the market. A higher price per kg of product can hereby be anticipated. The possible favourable financial consequences of the moment of harvesting are not included in this report.
A possible option for providing CO2 to the greenhouse is the production of CO2 in the greenhouse itself by means of a xe2x80x9cbacteria-richxe2x80x9d ground.
For the combined heating and power option with electric heat pump a study of the possibilities for CO2 storage is furthermore of importance for the use of the locally generated CO2.
From the viewpoint of a renewable energy provision, a market garden greenhouse can be considered a solar collector.
For maximum use of the annual insolation the surplus radiated solar energy (in the form of sensible and latent heat) will be collected on a xe2x80x9cwarmxe2x80x9d day and stored. Sufficient heat will have to be supplied from the store on a xe2x80x9ccoldxe2x80x9d day.
Fluctuations in the energy demand within a dayxe2x80x94caused by fluctuations in the outside climatexe2x80x94can also be compensated.
A stable inside climate requires a rapidly responding energy system.
The basis of the energy supply in the greenhouse system according to the present invention consists of a heat and cold-providing system in the form of a number of heat exchangers and air distribution units in the greenhouse. The heat exchangers have both a cooling and heating function. The air in the greenhouse is carried through the heat exchangers by means of fans; use can optionally be made of natural convection during heating.
The surplus heat is removed entirely to an aquifer in the summer. This takes place by active convection through heat exchangers. These heat exchangers are fed with cold water from an aquifer, see FIG. 1 wherein A to H are liquid flows (A and B are groundwater flows).
An aquifer is understood to mean a natural water source of often non-potable water which, stored in a sand layer, lies under the ground under pressure at a depth of roughly 80 m.
An aquifer is thus a kind of xe2x80x9cundergroundxe2x80x9d lake which cannot be termed xe2x80x9cgroundwaterxe2x80x9d since there is substantially no circulation of water in an aquifer.
Aquifers are often found in delta regions in North-West Europe.
The present invention preferably makes use of existing aquifers as energy store.
The aquifer can be limited in output capacity to the flow rate which can be processed by one doublet, consisting of one borehole for upward pumping of water and one borehole for downward pumping of water in a closed circuit.
The aquifer can be dimensioned such that the peak output of heat can be removed immediately.
By applying a day storage for both cold and warm water, the peak capacity for the cooling does not have to be extracted directly from the aquifer. In the night prior to a hot day a supply of cold water is stored which is large enough, together with the cooling from the aquifer, to remove the heat surplus at a high output during the day. The cold water extracts heat from the greenhouse and is then stored in a warm day buffer and in the following night removed to the aquifer. In this way the heat supply peak is removed sufficiently quickly and removed uniformly to the aquifer via buffering.
The day buffers can be embodied as two covered, uninsulated water basins such as are currently used as water store for watering. If necessary, these day buffers can also be placed underground. It is also possible to opt for a layered storage in one buffer.
The momentary heat surplus in the greenhouse can be removed in two ways:
directly to the aquifer
indirectly via day buffer to the aquifer.
A structural heat surplus in the aquifer can be removed in two ways:
cooling with cooling tower
supplying heat to third parties outside the greenhouse.
If a heat surplus occurs in the greenhouse, this heat will have to be stored in the aquifer. The quantity of heat to be stored determines the required storage capacity of the aquifer. This heat is used for heating in the winter.
In respect of the flow rate the aquifer has the smallest possible dimensions in order to keep investment costs as low as possible. This limits the storage capacity of the aquifer.
The maximum heat load can amount for a small number of hours in the summer to about 700 W/m2 or 7 MW/ha. This amount of heat is extracted from the greenhouse by means of heat exchangers.
The load duration curve shows that the large heat surplus occurs for only a small number of hours. In practice the peak of insolation is usually already excluded by closing a screen in the greenhouse when insolation is high. However, the light incidence is hereby also reduced, and therewith the production. Reduction of the insolation, and therewith the peak of the cooling demand, results in a decrease of energy consumption.
Possibilities of removing the heat from the greenhouse are:
Direct storage in aquifer. The flow rate is a maximum of about 150 m3/hour per pair of boreholes. At a temperature difference of 12xc2x0 C. this produces power output of about 2.1 MW. The aquifer will preferably consist of one pair of boreholes and thereby have power output of about 2.1 MW. The use of two day buffers is therefore recommended.
Via day buffer to aquifer.
At higher insolation than the above stated 2.1 MW, the surplus heat can be stored temporarily in a heat buffer.
This heat buffer is situated with a cold buffer in a closed circuit. These buffers can be placed both below and above ground in the form of water basins or storage tanks. There is a buffer with cold water which is cooled with water from the aquifer. The flow rate from the cold buffer is sufficiently great to realize a cooling power output of 7 MW at a temperature difference of for instance 12xc2x0 C. The pre-buffered quantity of cold water in this buffer and the continuous supply from the heat exchanger between aquifer and the buffers is sufficient to remove the whole heat surplus by cooling during a day with maximal insolation. The heated water is then stored in the warm day buffer. The water from this buffer is guided via a closed circuit through a heat exchanger where it relinquishes its heat to the cold water from the aquifer. The cooled water is stored in the cold buffer, the heated water is stored in the warm well of the aquifer. The size of the water buffers is determined by the daily heat surplus. From calculations based on hourly values of actual insolation, it is found that the heat surplus will amount to a maximum of about 200 GJ per day. It follows from calculations that about half thereof must be buffered, the other half already being removed to the aquifer during the day via the heat exchanger. At a temperature difference of 12xc2x0 C. the volume of the buffers will each amount to about 2000 m3 (for 1 ha. greenhouse). The warm water is pumped continuously through heat exchanger 8 (FIG. 1) to relinquish heat to the aquifer. The cold day buffer is refilled with the cooled water. This water is available the following day for cooling. During this day the empty warm buffer is then refilled with water which has extracted heat from the greenhouse. It can be deduced from the load duration curve that, of the 2645 hours with a heat surplus, cooling must take place with more than the capacity of the aquifer for about 35% of this number of hours (985 hours) The day buffers are used at least during these hours.
The surplus heat from the aquifer can be removed with a cooling tower. The warm water from the aquifer relinquishes heat via the heat exchanger to water which is cooled by the cooling tower. The water for cooling is brought into contact with ambient air by a spraying system. The ambient air has a lower temperature than the water for cooling and absorbs sensible and latent heat. The cooling water thereby cools to below ambient temperature and is fed back to the greenhouse where it once again absorbs heat. It is physically not possible to cool lower than the wet bulb temperature of the environment. Cooling in this manner is hardly worthwhile in the summer. In the winter however, this cooling method is worthwhile to cool the aquifer and to prevent a permanent warming of groundwater occurring. The surplus heat stored in the aquifer can also be removed by supplying it to other users outside the greenhouse.
The heat exchangers in the greenhouse can remove the heat surplus by cooling and simultaneously regulate the air humidity. The temperature of the supplied cold water is fixed at about 6xc2x0 C. By regulating the flow rate of the water and of the greenhouse air through the heat exchanger, the quantity of removed heat and moisture (sensible and latent heat) can be controlled.
Heating during the summer is provided by the warm water present in the day buffer and in the aquifer. Using a heat pump the temperature of the water from the day buffer and the aquifer is increased to about 40xc2x0 C. The heat required for this purpose is extracted from the warm water. The cooled water is pumped into the cold day buffer or into the aquifer.
The power output required for heating can be supplied wholly by the aquifer with one doublet. By applying the day buffers as communicating vessels for cooling and heating, even if the heat surplus is small, an energy-saving can be expected on the auxiliary energy.
Heating of the greenhouse is provided by the same heat exchangers as those used for the cooling. The required peak output for heating is lower than the peak output for cooling and is about 30% of the cooling power output. About half this output can be delivered by the heat pump (basic load) and half by the gas boiler (peak load). Another division of the basic load and the peak load of the heat demand is also possible.
On the basis of the heat demand of the greenhouse and the insolation, there is a surplus of heat on an annual basis. In the summer this heat is stored in the aquifer. In the winter the heat demand is not large enough to cool the whole aquifer to the original temperature. In order to prevent a structural warming of the aquifer occurring, the annual surplus of heat is preferably cooled with a cooling tower or supplied to third parties. The output of this cooling tower is preferably not larger than the output the aquifer can supply.
The cooled water from the aquifer is pumped to the cold well of the aquifer.
In the winter situation there is then substantially no buffering required.
When mains electricity is used, as much use as possible can be made of electricity during the night in order to keep energy costs as low as possible.
The climate in a greenhouse is determined by the insolation, the temperature, the relative air humidity and the CO2 concentration.
The insolation on the greenhouse cannot be influenced. The net insolation (the incoming insolation less the shadow of the greenhouse construction) must be as high as possible because the growth of many types of crop is proportional to the light incidence.
In Dutch conditions there is usually too much heat and too little light in the greenhouse. Ventilating windows are absent in a closed greenhouse so that there are fewer obstructions for the incident radiation. The amount of light available for growth is therefore slightly greater than in a traditional greenhouse.
The temperature in the closed greenhouse will preferably be between 20xc2x0 C. and 24xc2x0 C.
The highest possible temperature will be chosen during days with a heat surplus. This may be favourable for the growth of the crop and increase the transmission losses to the environment so that less heat has to be removed via the heat exchangers. The capacity of the heat exchangers moreover increases with an increasing temperature difference between the cold water from the aquifer and the air in the greenhouse.
The air humidity can preferably lie between about 70 and 90%. This is regulated by cooling the air in a heat exchanger to below the dewpoint. If in addition to dehumidifying there is also a heat demand, this heat can delivered directly by the heat exchanger. Latent heat is herein thus converted into sensible heat without heat being stored in the aquifer or the day buffer.
Pumping warm water downward to the aquifer and then pumping it back up again for heating purposes is preferably avoided.
The crop consumes considerable quantities of CO2. In contrast to the traditional xe2x80x9copenxe2x80x9d greenhouse, it is possible in a closed greenhouse to always set an optimal CO2 of about 1000 ppm.
The climate in a closed greenhouse can be managed much better than the climate in a traditional open greenhouse. The quality of the crop and the production can hereby be enhanced. An additional advantage is that the moment of harvesting can be influenced so as to choose a favourable time for delivery to the market.
An efficient regulation of cooling, heating, dehumidification and CO2 delivery is necessary in order to enable control of the climate in the greenhouse.
When there is a demand for heat in the greenhouse the air in the greenhouse is heated via the heat exchangers and a pipe system with warm water. The heat exchanger is fed with heat from the warm day buffer or the aquifer. This heat has too low a temperature for direct use and is increased in temperature by a heat pump to a maximum of about 40xc2x0 C. The lower this temperature, the higher the efficiency of the heat pump will be.
The heat can be generated by both active and passive convection. In passive convection, therefore without forced ventilation, no auxiliary energy is required for the air flow through the heat exchanger. The specific heat generation per m or m2 heat exchanger is however smaller than in active convection. Active convection during heating is assumed in the calculations of the total energy consumption. The electrical auxiliary energy amounts to 3% of the amount of exchanged heat.
The power output which the aquifer produces is roughly 2.1 MW, the heat pump adding about 500 kW thereto so that roughly 2.6 MW is available. This output is sufficient for the peak demand for heat in the greenhouse. If necessary, the warm day buffer can be used as heat source. This has the advantage that the amount of energy for pumping the water is lower than when an aquifer is used. If the heat pump is driven with a gas motor, the cooling water from the gas motor is also available for heating the greenhouse The temperature of this cooling water is about 80xc2x0 C. and thereby suitable for heating via a network of pipes in the greenhouse. This can be a network on the ground which also has the function of rails for carts used for harvesting.
Dehumidification is necessary to remove the moisture produced by the crop.
In the present invention two methods of moisture removal are recommended:
via condensation on the greenhouse deck (passive dehumidification)
via condensation on a cold surface (active dehumidification).
The dehumidification via the greenhouse deck costs no energy and can hardly be influenced.
A form of active dehumidification is necessary to enable control the air humidity. The heat exchangers are capable of removing sufficient moisture.
If there is a simultaneous moisture surplus and heat demand, it must be possible to convert latent heat into sensible heat. This process can take place with the heat pump. A part of the heat exchangers will cool and dehumidify the air while another part of the heat exchangers carries the extracted heat back into the greenhouse. Depending on the conditions in which this dehumidification takes place, this costs about 0.7 MJ/kg water.
The crop in the greenhouse consumes large quantities of CO2. The ideal concentration for the growth of the crop is about 1000 ppm, this concentration being particularly necessary at high insolation. The natural concentration in the outside air is about 350 ppm. In a traditional open greenhouse CO2 is supplied but the concentration only increases in the most important growth period, the summer, to about 500 ppm. Addition of more CO2 is pointless because ventilation takes place during high insolation to remove the surplus heat and moisture and the extra delivered CO2 thereby also disappears.
In a closed greenhouse according to the present invention the concentration of CO2 can however be brought to the desired level.
For tomato for instance a typical value of the CO2 consumption is 2 kg CO2 per kg dry matter product. The yield amounts to about 3-6 kg m2/year dry matter or a CO2 consumption of 6-12 kg m2/yr. Assuming a loss of 50% (first order estimate) about 12-24 kg CO2/m2/ year will have to be delivered.
Another possibility for delivery of CO2 is local production of CO2 by bacteria, for instance in the greenhouse soil.
The closed greenhouse is not ventilated with outside air. Some additional ventilation may be necessary to refresh the xe2x80x9ccontaminatedxe2x80x9d air in the greenhouse with xe2x80x9ccleanxe2x80x9d outside air. In this controlled ventilation the outside air must be supplied via filters. The chance of diseases through fungi and pollen is hereby greatly reduced.